Method of stabilizing alkali metal nitrate melts used in the heat treatment of metals



' ing saltpetre melts usedin the Patented July l8, lhd

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signer to I. G.

Farbenindustrie Aktiengesellschait, Fraort-on-the-in, Germany Nil Drawing. Application July 10,1937, Serial in Geny 31113722, 1936 This invention relates to a method of stabiliz heat-treatment of metals.

The heat-treatment of alloys, "especially aluminium alloys, on a commercial scale, is usually efiected with the aid of saltpetre baths. In order to obtain a melt of the greatest possible fluidity, even at comparatively low temperatures, the eutectic mixture (50/50) of potassium-nitrate and sodium nitrate, having a melting point of 218 C. is generally preiered. Working with these salts and mixtures thereof, however, isnot free from danger, since, owing to circumstances as yet unknown in detail, nitrate baths tend to undergo sudden decomposition of an explosive character, when in use. In this connection, an important part is undoubtedly played by the ready tendency of fused alkali metal nitrates to decompose into alkali metal nitrite and oxygen,

which may even lead to the formation of alkali metal oxide. It is also known that, within the working temperature range ofabout 200 to 500 C. nitrate melts attack the iron vessels, and also the work-pieces under treatment. With the Db?- ject of preventing such corrosion of the metal parts coming into contact with the saltpetre bath, it has already been recommended to add up to 2% of an alkali metal chromate, or dichromate, to the saltpetre melts. addition, which apparently establishes a superiicial passivation of the metal articles, prevents reaction between the saltpetre melts and the material, experience has shown that it does notobviate the sudden decomposition of thenitrate melts above referred to.

' It has now been ascertained in accordance with the-present invention, that decomposition of the" baths is reliably prevented by the addition of an .alkali-, or alkaline earth metal monochromate in quantities exceeding 2% to fused saltpetre baths, whereas an addition of dlchromates alone is ineflective for this purpose.

A similar, though less pronounced, efiect is produced byan addition of alkali or alkali earth metal tungstates, molybdetes, vanadates and manganates.

Based on the foregoing observations the present the invention.

llvhilst such an.

ing temperature of 300 C. are between 2 and 5%.; for 400 C. between 5 and 10%; and for 500 0.. between 10 and 20%, The same proportions, depending on the temperature, apply, when instead of alkali metal monochromate, the corresponding tungstate, molyb'date, vari'adate or manganate compounds are employed.

The following table, which specifies the nitrite content and the alkalinity (due to the formation oi Kim-both being representative of .the pro-,- gressiv'e decomposition of the bath-in a fused eutectic mixture of potassiumand-sodium nitrate, at a working temperature of 500C. ascertained after varying periods of employment of the bath, clearly shows the stabilizing effect of an addition, of monochromate in accordance with It has further been observed that the baths stabilized inaccordance with the present invention still exhibit a certain'small residual alkalinity which is attributable to a moderate reaction of the saline melt with-the work-pieces to be treated and also with any oil or other carbonaceous matter superficially adhering thereto, and which I alkalinity, although insuflicient in itself to cause corrosion of the aluminium alloys :-'under'.treat ment, nevertheless may be'objectionable, because it is liable to produce a discolouration on the surface of the-treated materialf moreover, on dissolution of the salt still superficially adheringto the treated workpieces in water used for subsequent quenching the latter becomes increasingly alkaline and may give rise to similar phenomena,

This residual alkalinity of the melts, irrespective as to whether it is due to alkali metal oxide, hydroxide or carbonate, can, in accordancewith the present invention .be avoided withnut' impairing the other favourable properties of the baths, by further adding to the melts containing monochromate in the above proportions. such an amount of alkali metal dichromate that'the free alkali still remaining in' the melts (which is .ascertainable by the usual; methods of analysis, e. g., by determining the pH value or a. 5% aqueous An excess of alkali metal bichr'omate should, how- 'ever, be carefully avoided, since it-would lead to decomposition phenomena in the melts.

The following examples are representative of the procedure to he followed in removing the residual alkalinity from baths according to the invention.

Example I' 5000 kgs. of a saline melt consisting of a mixture of equal parts by weight of potassium nitrate and sodium nitrate and containing in addition'8.66% oi sodium 'monochromate, after having been used i'or afortnight for the treatment of aluminium base alloys at temperatures between 310 and 320 0.15%

Accordingly, arr addition of 29.5 kgs. of potassium bichromate was made to this bath,- whereupon after two hours,- free alkali was no longer ascertain'able in the melt.

. Exani le'il 12,000 i k'gs.

which had, for the purpose of heat-treating aluminium alloys been. in use for a period of three yee'ks at temperatures between 320 and 330 0 were found to have it 0.28%

acquired a residual alkalinity (K O-i-N O) 2 '11 order to remove such residual alkalinity 132 :gs. of potassium bichromate were added to the nelt, samples of which, taken after four hours, howed a completely neutral reaction.

The residualalkalinity of the melts according o the present invention [me to time and re-adj1isted to zero by correponding additions of bichromate.

Seeing thateven a local supersaturation of the ielt withdichron'iate would result in a decompoltion of thenitrates present, itis advisable to icorporate ionochromate, by. previously intimately mixing ie monochromate and dichr omate'.in' the reuired relative proportions.

Although it might seem feasible, the use of ."iromic acid in place ordichromate, has been mud to beimpracticabl'e, since the chromic acid idergo'es decomposition under the temperature inditions obtaining... For. the purposes of the present invention, the hall metal compounds to be added to the ni-' ate baths may be replaced irth metals provided their limit of ie nitrate baths is not exceeded. Iclaim: I

solubility in 1. A method of stabilizing saltpetre melts for melt,

were found-to have a residual alkalinity oi ot a saline melt consisting of equal" parts by weight of sodium and potassium nitrate ind in addition 6.74% of sodium moncchromateshould be tested from the requisite amount of dichrom ate 'ith the melt, con ointly with the addition of or thetgmupconsis of states, vanadates, ranging between .2 and of the weight of the by-similar proporj I ons of the corresponding salts of the alkaline tures between about 350 and-150 a,iee,se4.. g earth metal mono-chromates,'molybdates, tungstates, .vanadates, and manganates, in a quantity" exceeding 2% butnot'exceeding about 209601 the weight of the melt, such quantity increasing withinv the range specified as the temperature of operation of 'the melt increases within the range from about 250 to about 550 0.

2. A process of stabilizing saltpeti'e melts for use in the heat-treatment of alloys at temperatures between abouti250 and 350 0., which comprises adding to such melt at least one substance of the group consisting of alkaliand alkalineearth metal mono-chromates, molybdates, tungstates, vanadates, and manganates in an amount ranging between 2 and'5% oftheweight oi the 3. A process of stabilizing saltpetre meltsffor -use in the heat-treatment of alloys at tempera- 0., which com-' tures between about 350 and 450 prises adding to such melt at least one substance or the group consisting of'alkaliand alkalineearth metal mono--chromates, states, vanadates, and manganates in' an amount ranging between 5 and of the weight of the melt. ....r.'

4. A process of stabilizing saltpetre melts 101' use in the heat-treatment of alloys at temperatures between about .450 and 550 0., which comprises adding to such melt at least one substance of the group consisting of alkaliand. alkalineearth metal mono-chromates, molybdates, tung-' states, vanadates, and manganates inan amount molybdates, tungranging between 10 and oi the weight of the melt,

5. A method of stabilizing saltpetremelts for use in the heat-treatment of alloys, which comprises adding to such melt at least one substance 01 the group consisting of alkaliand alkalineearth metal monochromates. molybdates, tongstates, vanadatcs, and manganates. in a quantity exceeding 2 but not exceeding about 20% of i the weight oi the meIt suchquantity increasing within the range specifled as the temperature or operation of the melt increases within the range from about 250 toabout1550 0., and also an amount of alkali metal 'di'c'hromate suiiicient to convert the free alkai'i present inthe melt into monociiromate in accordance with the equations:

melt,, and also an amount or alkali metal orchromate suincient to convert the freealkali present inthe '7. A process oi stabilizlng' saltpetre melts tor use in the heat-treatment 01 alloys at tempera- 0., which commelt. into monochrom'ate in ac-- cordance with the equations:

prises'adding to such melt at least one substance of the group consisting of alkaliand alkalineearth' metal mono-chromates, molybdates, tungistates, vanadates, and manganates in an amount.

ranging between 5 and 10 of the weight of the melt, and also an amount of alkali metal dichi'omate sufficient to convert the free alkali present .in the melt into monochromate in accordance with the equations:

present in the melt into monochromate in 'ac cordance with the equations:

K2CI'2O7-I-K2O =2K2Cl04 KzCrzOv-i-ZKOH =2KaCrO4+H20 and 9. A saline melt for use in the'heat-treatment of metals, substantially consisting of saltpetre .and containing at least one substance of the roup consisting of alkaliand alkaline-earth metal monochromates, molybdates, tungstates, vanadates, and manganates in a quantity exceedlng 2% but not exceeding'about 20% by weight of the saltpetre, such quantity increasingwithin the range specified as the temperature of operation of the melt increases within the range from about 250 to-about 550 C.

10.'A saline meltfor use in the, heat-treatment of metals at temperaturesibetween about '250 and 350 C., substantially consisting of saltpetre and containing between 2 and 5% by weight of the saltpetre of at least one substance of the roup consisting ofalkaliand alkaline-earth metal monochromates, molybdates, tungstates, vanadates, and manganates. I

11. A saline melt' for use in the heat-treatment of metals at temperatures between about 350 and 450 C., substantially consisting of saltpetre and containing between 5' and 10% by weight of the saltpetre' of at least. one substance of the 'group-consistlng of alkaliand alkalineearth metal monochromates, molybdates, tungstates,--vanadates, and manganates.

12. A saline melt for use in the heat-treatment of metals at temperatures between. about 450 and 550 0., substantially consisting of saltpetre and containingbetween 10 and 20% by weight of the saltpe'tre of at least one substance of the group consisting of alkaliand alkalineearth metal monochromates, molybdates, tungstates, vanadates, and manganates.

JOSEF MARTIN mom. 

